Porous materials are highly sought after for applications ranging from catalysis, water and air purification, electrodes, and for thermal insulation. The state-of-the-art materials for thermal insulation are silica aerogels. Aerogels are >95% air and only 5% solid silica. The aerogels provide high thermal insulation because of the air trapped inside the pores and minimum solid conduction. Air has a very low thermal conductivity, e.g., 0.024 W/m·K around room temperature and 1 atmosphere (K. Stephan and A. Laesecke, “The Thermal Conductivity of Fluid Air,” J. Phys. Chem. Ref Data, vol. 14, no. 1, pp. 227-234, 1985). When air molecules are trapped inside the small-sized pores, the thermal conductivity further decreases (A. Berge and P. Johans son, Literature review of high performance thermal insulation, Report in Buildings Physics, Chalmers University of Technology, 2012). Therefore, in porous materials, the trapped air contributes negligibly to the overall heat transfer and significant heat flow occurs through the solid network. As silica has a quite high thermal conductivity value (1.38 W/m·K), even 5% of silica network results in a substantial heat transfer. Efforts to further decrease the amount of solid silica in aerogels results in a very fragile material that becomes exceedingly difficult to handle and use in real applications.
Hollow silica particles have also been synthesized and considered for thermal insulation. The hollow particles contain a cavity filled with air and have a thin wall made of solid silica; therefore, the particles have an overall very low thermal conductivity (e.g., ˜0.025-0.03 W/m·K) (L. I. C. Sandberg, T. Gao, B P. Jelle, A. Gustaysen, Synthesis of hollow silica nanospheres by sacrificial polystyrene templates for thermal insulation applications, Advances in Materials Science and Engineering, pp. 483651-6, 2013). However, the lower thermal conductivity provided by hollow silica particles of the art still remains unacceptably high for many applications. Thus, there would be a significant benefit in further decreasing the thermal conductivity of such porous materials.